1. Field of the Invention
The present invention relates to a technique for detecting a jam of a recording medium in an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a printer, a copier machine and a facsimile machine, transportation means is disposed which transports a recording medium such as a paper and an OHP sheet along a predetermined transportation path. The transportation means generally has such a structure in which a recording medium is firmly held between a rotating member, such as a roller and a belt, and a roller. As the rotating member rotates in a predetermined direction, the recording medium is transported in the predetermined direction.
Such transportation means accompanies a jam, i.e., a phenomenon that a recording medium fails to be normally transported and instead gets wrapped around a roller or the like, which could damage not only the recording medium but the apparatus itself. Particularly in a fixing unit for heating and pressurizing a recording medium to which a toner image has been transferred and accordingly fixing the toner image, since a fixing roller which serves as transportation means also heats up a recording medium, the recording medium can easily get wrapped because of curling of the recording medium or molten toner. In an effort to prevent a jam-induced damage to the apparatus, a number of jam detecting techniques for immediately sensing a jam upon occurrence of the jam have been proposed.
FIGS. 1A and 1B are principle drawings which show one example of a jam detection according to a conventional technique. The jam detecting technique illustrated in FIGS. 1A and 1B is a technique for detecting a jam of a recording medium at a fixing roller in an image forming apparatus described in Japanese Patent No. 2,858,441. According to the jam detecting technique, as shown in FIG. 1A, there are sensors disposed one ahead of and the other behind a fixing roller R on a transportation path FF, that is, there are a paper feed sensor Sa ahead of the fixing roller R and a paper discharge sensor Sb behind the fixing roller R. Occurrence of a jam is detected from the timing of changes of outputs from these sensors. These sensors Sa and Sb are each for sensing whether there is a recording medium SS such as a paper at the position of the sensor, and output an L-level when there is a recording medium there but an H-level when there is not a recording medium there.
During normal transportation of the recording medium SS along the transportation path FF from the left-hand side in FIG. 1A, the output from the paper feed sensor Sa changes to the L-level upon arrival of the leading edge of the recording medium SS at the paper feed sensor Sa but to the H-level upon arrival of the trailing edge of the recording medium SS at the paper feed sensor Sa. Meanwhile, the output from the paper discharge sensor Sb changes similarly to the output from the paper feed sensor Sa, with a certain delay in time. On the contrary, in the event that the recording medium SS gets wrapped around the fixing roller R as denoted at the reference symbol J in FIG. 1B, the leading edge of the recording medium SS moves backward, and the output from the paper discharge sensor Sb changes to the H-level before the output from the paper feed sensor Sa does. Therefore, in such an instance, that is, when the output from the paper discharge sensor changes the H-level before the output from the paper feed sensor changes to the H-level, driving of the fixing roller R is immediately stopped, determining that a jam has occurred.
However, the conventional technique described above has the following problem. According to this technique, since occurrence of a jam is determined only in accordance with which one of the two sensor outputs changes before the other one does, it is not possible to detect occurrence of a jam which has taken place after the trailing edge of the recording medium SS has moved passing the position of the paper feed sensor. In addition, the arrangement of the sensors imposes a restriction upon the size of the recording medium with which it is possible to detect a jam. That is, as for a recording medium which is shorter than the length of the transportation path between the two sensors (i.e., the length La+Lb shown in FIG. 1A), since the paper feed sensor output returns to the H-level before the paper discharge sensor output changes to the L-level, it is not possible to detect a jam which occurs later than this by the method described above.